The present invention relates to a method and apparatus for determining the dispersion stability of a liquid suspension containing particles. More particularly, the present invention relates to a method and apparatus for determining the dispersion stability of a liquid suspension through analyzing the light scattering properties of the liquid suspension.
A common problem in preparing liquid suspensions containing insoluble particles is that a liquid suspension often becomes unstable and phase separates over time. For example, if the insoluble particles are denser than the liquid, the particles will settle downward, leaving a layer of solids towards the bottom of the suspension, and a liquid layer towards the top of the suspension. In the paint industry, this phase separation phenomenon is sometimes referred to as xe2x80x9csyneresis.xe2x80x9d Dispersion instability, however, is not just limited to paints. For example, dispersion instability often occurs in liquid suspensions such as cosmetic compositions for application to the skin, hair, and nails; topical pharmaceutical compositions; pesticidal or agricultural compositions; mining compositions such as drilling muds; and aqueous or solvent based coatings such as adhesives, polishes, waxes, and coatings for substrates such as wood, reconstituted wood products, concrete, asphalt, fiber cement, stone, marble, clay, plastics, paper, cardboard, leather, textiles, and metal (ferrous as well as non-ferrous). Dispersion instability in these liquid suspensions may be evidenced, for example, by insoluble particles rising to the top of the suspension, or the insoluble particles settling to the bottom of the liquid suspension, depending on the density of the insoluble particles relative to the liquid.
Dispersion instability in a liquid suspension can be caused by a variety of factors. For example, dispersion instability can be caused by not properly dispersing the insoluble particles in the liquid. Dispersion instability can also be caused by subjecting the liquid suspension to external conditions such as shear, heat, or freeze-thaw cycles. Also, dispersion instability can be caused through changes in the particle size of the insoluble particles. For example, additives in the liquid suspension can cause insoluble particles in the liquid suspension to combine together through such mechanisms as bridging flocculation.
A common technique that formulators use for testing the stability of a liquid suspension is to store a sample of the liquid suspension for a predetermined time under predetermined conditions and to visually inspect the sample after the predetermined time to determine if the liquid suspension has phase separated. However, this technique has the disadvantage that it takes time, often more than several hours, to determine whether the liquid suspension is stable. Additionally, the test is subjective in that an operator must visually determine whether phase separation has taken place.
Another technique used for detecting the stability of a liquid dispersion is disclosed in U.S. Pat. No. 5,783,826 to Meunier (hereinafter xe2x80x9cMeunierxe2x80x9d). Meunier discloses a method and apparatus for analyzing phases of a multi-phase mixture by emitting electromagnetic radiation towards a tube containing the liquid suspension and detecting the radiation back scattered by the liquid dispersion over the length of the tube to detect changes in phases over the length of the tube. Thus, as in a conventional dispersion stability test, actual phase separation must begin to occur for the method and device of Meunier to detect dispersion instability.
The present invention provides a method and apparatus for determining the dispersion stability of liquid suspensions of insoluble particles which does not rely on the liquid suspension actually beginning to phase separate.
In one aspect, the present invention provides a method for determining the dispersion stability of a liquid suspension including the steps of providing a liquid suspension comprising insoluble particles; exposing the liquid suspension to a light beam having at least one wavelength of light while maintaining the insoluble particles in the liquid suspension uniformly suspended; determining the quantity of light scattered by the liquid suspension; and analyzing the quantity of light scattered to determine the dispersion stability of the liquid suspension by detecting the presence of modified insoluble particles based on the quantity of light scattered by the liquid suspension.
In another aspect, the present invention provides an apparatus for determining the dispersion stability of a liquid suspension including light generating and detecting equipment for exposing a liquid suspension to a beam of light having one or more wavelengths of light and for determining the quantity of light scattered by the liquid suspension; means for maintaining insoluble particles in the liquid suspension uniformly suspended while exposing the liquid suspension to the beam of light; and an analyzer for analyzing the quantity of light scattered by the liquid suspension at the one or more wavelengths to determine the dispersion stability of the liquid suspension by detecting the presence of modified insoluble particles.